1. Field of the Invention
The present invention relates to a sheet conveying apparatus which is provided with curl correcting means for correcting a curl in a sheet and which is applicable to an image forming apparatus or the like.
2. Related Background Art
Conventionally, in an image forming apparatus, such as an electrophotographic copying machine or a printer, a toner image formed on a photosensitive drum, which serves as an image bearing member, is transferred to a sheet, and then the toner is fixed to the sheet by a fixing device provided with a fixing roller and a pressure roller held in pressure contact with the fixing roller, thereby fixing the toner image to the sheet as a permanent image. Further, in a certain type of image forming apparatus, sheets to which images have been thus fixed are sorted out by a finisher or the like so that post-processing may be easily conducted.
In such an image forming apparatus, to achieve a reduction in the size of the apparatus main body, the size of the fixing device is reduced, for example, by providing a built-in heater solely in the fixing roller, which is arranged on the image-surface side, and reducing the diameter of the pressure roller. However, in the fixing device constructed as described above, heat is imparted solely to the image-surface side, so that there is a difference in water content between the front surface and back surface of the sheet, with the result that the sheet is curled.
When the sheet is thus curled, faulty sheet transport such as jamming may be caused or the finisher etc. may not be able to function in the case where the sheet passed through the fixing device is caused to be contained in the finisher, discharge tray or the like.
To eliminate this problem of curling, a curl correcting device, such as shown in FIG. 12, has been invented, and this curl correcting device is arranged between the image forming apparatus and the finisher or the discharge tray.
Referring to FIG. 12, the curl correcting device is composed of a soft roller 51 formed of sponge or the like and a hard roller 52. The hard roller 52 is pressed against the soft roller 51 directly or through a belt (not shown) to thereby thrust the hard roller 52 into the soft roller 51, forming a curved nip N between the two rollers 51 and 52.
The sheet P is passed through this nip N while being urged so as to be wound around the hard roller 52 to thereby impart rigidity to the sheet P, thereby correcting the curl of the sheet. When the soft roller 51 is, for example, a sponge roller having an outer diameter of approximately xcfx8620 and a rubber hardness of 25xc2x0 (ASKAR C), the outer diameter of the hard roller 52 is not more than xcfx8612, and, more preferably, xcfx868 or xcfx866, whereby appropriate curl correction is possible.
In this curl correcting device constructed as described above, the requisite pressure force with which the hard roller 52 is pressed against the soft roller 51 is approximately 50 to 60 N in total. When such pressure force is generated, deflection is apt to be created in the middle portion in the longitudinal direction of the hard roller 52, which is supported at both ends. To prevent this deflection of the middle portion in the longitudinal direction, a support member for preventing deflection is provided underneath the middle portion in the longitudinal direction of the hard roller 52. Generally speaking, silicone oil is applied to the sheet P passed through the fixing device so that the toner and the sheet P may not adhere to the fixing roller.
However, a conventional image forming apparatus provided with such a curl correcting device has a problem in that, when the curl correcting means is arranged directly on the output side of the fixing device, the length of the apparatus main body in the sheet conveying direction increases, resulting in an increase in the size of the apparatus main body. Further, in the conventional construction, in which the hard roller is pressed against the soft roller, a large torque is required for driving, with the result that the size of the curl correcting device is rather large. When the pressure force is small, it is impossible to effectively correct various types of curl.
The present invention has been made in view of the above problems. It is accordingly an object of the present invention to provide an image forming apparatus which does not involve an increase in apparatus size and which is capable of correcting curl effectively.
According to the present invention, there is provided an image forming apparatus in which an image formed on a sheet by an image forming portion is fixed to the sheet by a fixing device before the sheet is discharged, the image forming apparatus comprising a discharge path for discharging the sheet, a separation path separated from the discharge path, a surface reverse path for causing the sheet to move toward the discharge path after reversing the surfaces of the sheet conveyed to the separation path, and curl correcting means provided in the surface reverse path and adapted to correct any curl created in the sheet when it is passed through the fixing device.